Automatic bed level control for furnaces

ABSTRACT

An automatic bed level control system is provided for furnaces which burn a material on a bed, such as chemical recovery furnaces and the like. Optical sensing means sense light emitted by the burning of the bed material and provide a signal which varies as a function of the intensity of the sensed light. The sensing means are structured and oriented to respond to light emanating only from a particular, vertically limited zone in the furnace, such that variations in the intensity of the sensed light are indicative of variations in bed level. The signal provided by the sensing means is then used to regulate some bed level controlling parameter, as for instance primary air flow.

United States Patent [1 Gilbert AUTOMATIC BED LEVEL CONTROL FOR FURNACES[75] Inventor: Lyman Francis Gilbert, Somers,

Conn.

[73] Assignee: Combustion Engineering, Inc.,

Windsor, Conn.

22 Filed: Dec.l0,1973

21 Appl. No.: 423,645

[ 5] Nov. 12, 1974 Herbst 122/7 Freiday llO/7 [5 7 ABSTRACT An automaticbed level control system is provided for furnaces which burn a materialon a bed, such as chemicalrecovery furnaces and the like. Opticalsensing means sense light emitted by the burning of the bed material andprovide a signal which varies as a function of the intensity of thesensed light. The sensing means are structured and oriented to respondto light emanating only from a particular, vertically limited zone inthe furnace, such that variations in the intensity of the sensed lightare indicative of variations in bed level. The signal provided by thesensing means is then used to regulate some bed level controllingparameter, as for instance primary air flow.

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BACKGROUND OF THE DISCLOSURE The invention relates to furnaces whichburn fuel in a bed. More particularly it relates to a method andapparatus for controlling the depth or level of the bed in a furnace.More particularly still it relates to a method and apparatus forutilizing the light associated with burning on the bed to regulate avariable which affects bed depth.

The control of the level or depth of the bed in furnaees such aschemical recovery furnaces, incinerators and the like may be quiteimportant in optimizing the burning function of the furnace. The bed isthe accumulation of various fuels and other materials which reside on afloor or grate in the furnace and is burned, either to reduce itsoverall solid mass or to effect certain chemical changes or both. Aparticularly good example of the need to control bed depth is seen inchemical recovery furnaces.

The operation and control of chemical recovery furnaces for the recoveryof chemicals from kraft black liquor is a difficult problem from thestandpoint of both maximizing the efficiency of the process, andminimizing air pollution and the explosion safety hazards. One factorwhich makes the operation and control difficult is that the composition(particularly solids content) of the black liquor varies from time totime. Another factor is the variety of chemical reactions which takeplace in the furnace. Several important changes in the black liquor takeplace in the chemical recovery furnace, the first of which is thevaporization of the water which has not been previously removed bydirect and/or indirect evaporators upstream. The carbonaceous materialin the black liquor then burns in a pile on the hearth while theinorganic materials fuse and form smelt in the furnace. Reducingconditions are maintained in the lower hearth region of the furnacewhile oxidizing conditions are maintained higher up in the furnace. Someof the primary reactions which take place in the chemical recoveryfurnace are as follows:

The control of the operating conditions in the lower furnace controlsthe reduction of the sodium sulfate to the sodium sulfide and conversionof sodium carbonate to sodium oxide and sodium oxide to sodium vapor.

The amount of sodium vapor formed byreduction in turn controls theconcentration of sulfur oxides and the sodium sulfate and sodiumcarbonate fume emitted in flue gas from the furnace. Proper control ofthe burning, and particularly reducing, conditions is thereforenecessary not only to regulate the efficiency of the production ofsodium sulfide from sodium sulfate, but also to regulate furnaceemissions. Increased steam production is an additional benefit ofcorrect operation.

In general, a high bed burning temperature and a relatively deep charbed in the furnace are desirable to maximize reduction efficiency and todecrease sulfur oxide and particulate (fume) emission. However, furnaceoperators are often reluctant to operate with a deep char bed because itrequires continuous attention and adjustment of the primary air dampers.Another disadvantage of operating with an excessively deep char bed isthat the air ports around the periphery of the furnace can becomeblocked, causing a blackout when an undercut char bed falls over towarda furnace wall. A further problem with deep bed operation is that it isoften difficult to determine by mere visual observation the exact heightof the bed and, therefore, difficult to tell if the bed is getting toodeep. For these reasons, many operators operate the furnace underconditions which will maintain a bed level which is excessively low,thereby impairing the reduction efficiency and increasing the carry-overof char in the gas stream which forms low melting slag deposits on heattransfer surfaces downstream. Operation with a low bed requires littleoperator attention, but produces low chemical reduction efficiencies.

It is desirable, therefore, to operate the char bed at a depthsufficient to obtain good reduction efficiency, but not so deep that thechance for collapse and blackout occur. This preferred bed level willtend to have an angle of repose of some 40 to with its base some 8-12inches below the primary air ports. The angle of repose of anexcessively high bed will exceed 50 and its base will be only an inch ortwo below the primary air port. When the angle of repose of a bed isless than 40 and its base more than 12 inches below the primary airports, the bed is too low for the most efficient reduction.

In a US. Pat. Application entitled MONITORING CHEMICAL RECOVERY FURNACEby I-I.W. Nelson filed on Apr. 23, 1973, Ser. 353,828, there wasdisclosed a technique and means for monitoring the sodium vapor producedby the char bed as an indirect indication of the efficiency of thereduction of Na SO to Na S. An optical device sensitive to the photonenergy emitted by burning sodium and relatively insensitive to otherphoton energy from the burning char bed was used to measure the rate ofsodium vapor production. The application further mentioned positioningthe optical device above the char bed to look downward thereon and thata drop in the bed level would produce a change in output reading of theoptical device.

Similarly, in other types of furnaces. such as incinerators and thelike, which also burn a fuel on a bed to effect a desired chemicaland/or mass change, it is necessary to control the depth of the bed foroptimum reduction or oxidation efficiency.

SUMMARY OF THE INVENTION The present invention involves a technique andmeans for controlling the level or depth of the bed in a furnace, suchas a chemical recovery furnace. As several variables, such as primaryair flow, and/or fuel temperature and fuel flow and/or fuel temperatureare effective to control the bed level, the invention provides means forand a method of sensing the intensity of light emanating from thefurnace, particularly light only from a zone of limited vertical extent,and using it to provide a control signal which is operative to vary oneor more of the bed level controlling variables as a function of thesensed lights intensity.

Optical sensing means, preferably having a limited vertical sight anglewithin the furnace, is effective to control one or moreofthe bed levelcontrolling variables, as for instance air flow. This may be done by controlling dampers in primary air ducts. The sensing means is preferablypositioned to view the bed and furnace interior through the primary airports. The sensing means may be responsive to light of one or morewavelengths emanating from the furnace as a result of burning gases fromthe bed, though selective sensing of light of a single wavelength, suchas emitted by excited sodium atoms in a chemical recovery furnace, maybe preferred.

Plural optical sensing means positioned to view the furnace interior atvarious positions around the bed, each associated with a differentregional primary air supply and damper,'provide a preferred means forcontrolling bed level.

For a better understanding of the invention, its operating advantagesand the specific objects obtained by its use, reference should be madeto the accompanying drawings and description which relate to a preferredembodiment of the invention.

. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic elevationalview of a black liquor chemical recovery furnace incorporating thepresent invention.

FIG. 2a is a somewhat diagrammatic elevational view of the furnace withthe char bed at a preferred level.

FIG. 2b is a somewhat diagrammatic elevational view of the furnaceshowing an excessively high char bed.

FIG. 20 is a somewhat diagrammatic elevational view of the furnaceshowing an excessively low or shallow char bed.

FIG. 3 is an enlarged view of a portion of FIG. 1, schematicallydepicting the optical sensor, the primary air damper and the controlcircuitry and actuator connected therebetween.

FIG. 4 is an elevational sectional view taken along line 4--4 of FIG. 3to approximate the view of and through the primary air ports by theoptical sensor.

FIG. 5 is a schematic cross-sectional view taken along line 55 of FIG.1.

DESCRIPTION OF THE PREFERRED EMBODIMENT While the present invention isgenerally applicable to furnaces of different types which burn amaterial on a bed, it finds particular utility in use in chemicalrecovery furnaces. Accordingly, the invention will be described asapplied to a chemical recovery furnace, but finds applicability as wellwith other furnaces which maintain beds.

FIG. l-illustrates a chemical recovery furnace which is typical ofchemical recovery furnaces used for the processing of black liquor. Thewalls of this furnace are lined with steam generating tubes 12 whichform a part of the heat exchange surface of the chemical recovery unitwith there being additional heat exchange surface identified generallyat 14 in the upper region of the unit.

Black liquor obtained from the kraft pulping process and/or other sodiumbased pulping processes which has been processed by evaporation to thedesired solids content is introduced into the furnace 10 through thenozzles 16. The liquor thus sprayed into the furnace descends downwardlytowards the furnace bottom passing through rising combustion gas suchthat a majority of the moisture in the liquor is immediately evaporated.The solid particles fall downwardly through this rising combustion gasstream and form a pile or char bed 18 on the hearth or furnace bottom20. A portion of the combustibles are consumed during this descentthrough the furnace with remaining combustibles being consumed in thechar bed 18. The noncombustibles, inorganic chemicals, are smelted anddecanted from the furnace through the discharge spout 22.

Combustion-supporting air is introduced into the furnace at twolocations. The primary air is introduced through nozzles or ports 24spaced relatively close to the bottom while the secondary air isintroduced through the nozzles or ports 26 located above the liquornozzles 16.

In addition to the reduction of sodium sulfate in the bed according toreaction (1 l. Na SO 2C Na s 2CO the sodium carbonate is thermallydecomposed according to the reaction 2. Na CO Na O (solid) +CO Thereverse reaction is depressed due to the escape of the CO and also tothe immediate use of Na O in reaction (3) 3. Na O C Na (vapor) CO.Sodium oxide is a thermally stable, relatively non-volatile solid with aboiling point of 2,320F. This is well above bed temperatures which arenormally about 1,500F to 2,000F. The elemental sodium produced inreaction (3) by contrast, has a low boiling point of l ,6 l 8F and thusreadily volatilizes from the bed. It is a very reactive substance andquickly burns according to reaction (4) just above the bed giving offsubstantial heat and a bright yellow light,

4. Na (vapor) 1/20 N3 0 (fume) AH Light. 0 I

This yellow light comes from the thermal excitation of sodium atoms inthe extremely fine, hot solid particles of Na O fume. It has sharp peaksof intensity in the 5,890 Angstrom wavelength region of the visiblelight spectrum. This wavelength is characteristic of sodium either inthe elemental or compound form. Other gases or vapors may also beevolved, contributing their characteristic spectral emissions to thelight emitted on and above the char bed. This light may be both inand/or near the visual range and is typified by, though not limited to,the sodium spectral emissions. This light generally originates at thesurface of and just above char bed 18 and is represented in thedrawings, such as FIGS. 2a, 2b and 20, by the shimmering waveform 27.Light waveform 27 indicates the region in which the light originates andthe amplitude of its oscillations provide a relative indication of thelight intensity thereat, the light generally covering most of the bedand being of greatest intensity near the center and fading away from thecenter at the cooler side areas.

FIGS. 2a, 2b and 2c respectively, depict char bed 18 having a preferredlevel or depth, an excessively high level and an excessively low level.As previously described, the preferred bed will have an angle of reposebetween 40 and 50 and its base will be some 8-12 inches below theprimary air ports 24; the excessively high bed of FIG. 2b will have anangle of repose exceeding 50 and its base will be at or an inch or twobelow ports 24; and the low bed of FIG. 2c will have an angle of reposeless than 40 and its base will be 12-18 inches or more below ports 24.

The meter 28 for use in the present invention to receive and indicatethe intensity of the photon energy emitted by burning gas or gases frombed 18 is an optical device substantially as described in theaforementioned application. Typically, meter 28 will include aphotodetector of some well known type, such as a photoresistor,photodiode, phototransistor, photomultiplier or the like. Thephotodetector may be of a type responsive to a particular portion of thevisible light spectrum, such as the portion including the sodiumspectral emissions or it may be broader in range to include the fullvisible spectrum and possibly beyond. Appropriate filters, prisms,diffraction gratings or the like might also be used to restrict thesensed light to a certain portion of the spectrum. Because most of thelight in chemical recovery furnace is in the sodium region of thespectrum, it is desirable that the meter 28 be responsive to such light.

Meter 28 is positioned and oriented such that it views a particular zonewithin furnace 10. The horizontal or azimuthal extent of the field ofview of a meter 28 is not particularly critical, but should include arepresentative portion of char bed 18. At least one meter 28 should bepositioned at each of the four sides of furnace 10, and preferably onewith each of the primary air ducts 29, as seen in FIG. 5 and describedhereinafter. The vertical field of view of a meter 28 is, however,somewhat more critical and should be restricted to a zone, indicated bydotted lines 30 in FIGS. 2a, 2b and 20, which does not include the fullvertical range of the light waveform 27 between the high bed level ofFIG. 2b and the low bed level of FIG. 2c. It is desirable that thepositioning of meter 28 be such that changes in the bed level causemaximum changes in the intensity of the light sensed by it. This may beaccomplished by positioning and orienting meter 28 such that it sightsin a substantially horizontal direction and its vertical angle or extentof view is limited to satisfy the requirement mentioned above.

In the preferred embodiment, the upper side of the field of view 30 issuch that most of the light waveform 27 for a high bed, as in FIG. 2b,is excluded from view; more of the light waveform 27 for anintermediate, and in this case, bed level as in FIG. 2a, is included inthe vertical field of view; and still more of the light waveform 27 fora low bed level, as in FIG. 20, is included in the vertical zone orfield of view. Stated another way, the field of view of meter 28 is suchthat an indication of low sensed light intensity is indicative of a highbed level; and indicationof high sensed light intensity is indicative ofa low bed level; and an indication of an intermediate sensed lightintensity indicates a bed level in between. The indication may becalibratd against known or observed bed levels. While other conditionsaffect light intensity somewhat at any given bed level, the effects areminimal and the control response to be discussed is in the correctdirection.

The meter 28 is preferably mounted on primary air duct 29 such that itis outside observation window 32 and is oriented to sight through thewindow and the air ports 24 at char bed 18. In this configuration theair ports 24 determine the zone viewed within furnace 10. Similarly,because calibrated of the spacing between meter 28 and air ports 24 andthe vertical extent of the air ports in a typical furnace 10, the upperextent of the field of view in the furnace is held below most of lightwaveform 27 in the high bed condition of FIG. 2b. Air

duct 29 and air ports 24 serve as a form of collimator for meter 28,however it will be appreciated that other collimating means might beused and that the meter might be located elsewhere with similarlimitations on the zone viewed within the furnace. Light conductors.such as optical pipes or fibers, might be used to conduct light from thefurnace wall to a remote sensor.

There are several parameters which affect the depth of bed 18, as forinstance, primary air flow and/or temperature, liquor flow and/ortemperature, and/or percent of solids to water in the liquor. In anincinerator, the liquor would be replaced with refuse. While any or allof these parameters may vary somewhat, either inadvertantly or throughhuman operator action, it is desirable to select one, as for instanceprimary air flow, which may be automatically controlled by a signal frommeter 28 to compensate for changes in the other parameters and maintainthe bed at a desired level.

Primary air flow is a parameter which is largely responsible forcontrolling the level of bed 18 and is relatively easy to vary. When theprimary air flow is increased, the bed level tends to decrease and whenthe primary air is decreased, the bed level tends to increase. A damper34 in each of the primary air ducts 29 is operative to control the rateof flow of primary air through its associated air duct. While suchcontrol has previously been of a manual nature and in response tooccassional viewing of the bed 18 through window 32 by the operator, theinvention utilizes meter 28 as means for providing a signal which servesto automatically control an actuator connected to damper 34. In thismanner, accurate, continuous automatic control of the level of char bed18 is effected.

Referring to FIG. 3, there is a somewhat diagrammatic schematical viewof furnace 10, optical meter 28, primary air damper 34, damper actuatingmeans such as cylinder 54 and rod 58, and the control circuitryintermediate the meter and the actuating means. Meter 28 is shown asincluding a photodetector represented by variable resistor 38.Typically, the photodetector will exhibit a characteristic, i.e.,resistance change, which varies as a function of the intensity of lightsensed. The detector might alternately exhibit a voltage or currentchange. While the photodetector might be part of a bridge circuit, it isshown here singly providing a light intensity signal for simplicity. Thesignal indicative of the intensity of the sensed light is applied, viaconductor 39, as one input to a conventional motor operator control unit40. Control unit 40 typically includes an operational amplifier and apower switch. A set point signal is developed from potentiometer 42 andis applied as another input to control unit 40. The set point signal andthe light intensity signal will typically be of opposite polarity andthe magnitude of the set point signal will be set at that valuecommensurate with the magnitude of the signal from meter 28 duringoptimun or preferred char bed level conditions.

The power switch portion of control unit 40 is operatively connected tothe operational amplifier and includes a three position, three terminalswitch, one terminal being connected by conductor 44 to the solenoid ofvalve 46 and another terminal being connected by conductor 48 to thesolenoid of valve 50. The common of the switch being connected to asource of power. Both valves 46 and 50 are connected in a pneumaticcircuit between air supply 52 and pneumatic cylinder 54. A pressurereducing valve 56 is also included in the pneumatic circuit. Valves 46and 50 are normally closed when the switch is in its neutral thirdposition andare opened only when the switch closes the electricalcircuit to the particular valve solenoid, only one valve being open at atime.

Cylinder 54 is pivotably mounted to some support structure at one endand piston rod 58 extends from the other end. Piston rod 58 is connectedto damper actuating linkage 60 which in turn is connected to damper 34.Linkage 60 may, at least in part, comprise the existing manual controllinkage for the damper. Movement of piston rod 58 results in rotation ofdamper 34, to proportionally control the air between fully open (maximumair flow) and fully closed (no air flow). Means other than pneumaticcylinder 54, suchas a motor or diaphragm, might be used to actuatedamper 34.

Position indicating linkage 62 is rigidly connected at one end to pistonrod 58 and at the other end to a rotary potentiometer 64. Potentiometer64 has a voltage applied thereto and its wiper is connected throughcapacitor 66 by conductor 68 to another input of the operationalamplifier of control unit 40. The signal provided by potentiometer 64and capacitor 66 is a form of derivative feedback. The polarity of thevoltage on potentiometer 64 and the direction of rotation of thepotentiometer relative to the direction of motion of pis ton rod 58 areselected such that a feedback signal voltage is applied to theoperational amplifier input during actuation of cylinder 54. Thisfeedback signal is temporarily effective to cancel the differencebetween the set point signal and the control signal from meter 28,resulting in closure of both valves 46 and 50 and cessa tion of movementof piston rod 58. The derivative feedback produces an integrating actionfor the damper position.

Considering now the operation of the automatic bed level control systemwith the described positioning of meter 28,- when the level of char bed18 is higher than preferred, as seen in FIG. 2b, the intensity of lightfrom the zone of the bed sensed by meter 28 will be relatively low,causing an increase in resistance of photodetector 38 and a decrease involtage applied by conductor 39 to control unit 40. This decreasedvoltage is less in magnitude than the set point and an error signalresults, closing the switch to conductor 44 and opening valve'46.Pressure reduction valve 56 provides a lower pressure to the left sideof cylinder 54 than the right and so piston rod 58 moves leftward toopen damper 34 somewhat. The movement of rod 58 is effective, throughlinkage 62, to rotate potentiometer 64 a certain amount, changing thevoltage applied to capacitor 66 and resulting in a dv/dt voltage appliedto control unit 40 to momentarily cancel the input error to theoperational amplifier, thereby opening the circuit to the solenoid forvalve 46 and thus closing the valve. This stops the movement of pistonrod 58 and damper 34. However the dv/a'i voltage is removed whenmovement of rod 58 stops, allowing any remaining error to once againopen valve 46. This operation is repeated with attendant short steps ofdamper 34 until the error signal is removed by a change in bed level andsensed brightness or until the damper is full open, the latterultimately producing the former.

Conversely, when the char bed level is below that preferred, as in FIG.20, the increased intensity of the sensed light so decreases theresistance of photodetector 38 that the voltage applied by conductor 39to control unit 40 exceeds the set point signal voltage in the oppositedirection, causing an error signal of opposite sense to actuate thepower switch and close the circuit with the solenoid of valve 50 to openthe valveaThis vents the right side of cylinder 54, allowing piston rod58 to move rightward to close damper 34. Again, but in a reverse sense,potentiometer 64 and capacitor 66 apply a dv/dl voltage to the controlunit 40, such that the closing motion of the damper is accomplished inshort steps. Again, this operation continues until the error signal isremoved or the damper is fully closed.

The net result of this control during normal operation is to maintainbed 18 at or near the level desired, as dictated by the set point. Thiscan usually be accomplished with but a minimum of displacement of thedamper.

While the bed level control system just described was associated with asingle primary air duct 29, it will be appreciated that at least onesuch arrangement would exist at each of the four sides of the furnace10. If there is more than one duct 29 at each side of the furnace, onemeter 28 per side might be used to control the several dampers 34 forthat side. Alternatively, as seen in FIG. 5, a separate meter 28 mightbe associated with each duct 29 and its damper 34. Still further, ifregional control of char bed depth is not required, the control signalsfrom the several meters 28 may be averaged and the resultant used tocontrol all of the dampers 34.

Further, while the described embodiment of the invention was in use in achemical recovery boiler, it will be similarly applicable toincinerators and other furnaces having beds for sensing light intensityas an indicator of bed depth and regulating a control variableaccordingly. In furnaces in which the dominant source of light comesfrom burning carbon and hydrocarbons on the bed, the photodetector mightbe similarly oriented to view the bed with a vertically limited angle ofview, but might respond to a broader range of optical wavelengths orelse that or those characteristic of burning carbon or hydrocarbons.Further, some other parameter or parameters than just primary air mightbe controlled to regulate-bed depth, however the latter is generallypreferred.

While the preferred embodiment of the present invention has beendescribed herein, it should be understood that the description is merelyillustrative and that variations and modifications can be made thereinwithout departing from the spirit and scope of the invention as recitedin the following claims.

What is claimed is:

1. In a furnace in which a material is burned on a bed, emitting light,and in which the level of said bed is a function of a control variable,means for controlling the level of said bed comprising:

a. means for sensing said emitted light and providing a signal whichvaries as a function of the intensity of said sensed light and isindicative of the level of said bed; and

b. means responsive to said signal for controlling said control variableto control said bed depth.

2. The apparatus of claim 1 wherein said sensing means sense light onlyfrom a particular zone in said furnace such that the intensity of saidsensed light is related to the level of the bed with respect to thepoint of measurement.

3. The apparatus of claim 2- wherein said particular zone is of limitedvertical extent within said furnace.

4. The apparatus of claim 3 wherein primary air is supplied to saidfurnace for burning said material and comprises said control variable;and said signal responsive means include means for varying the flow ofsaid primary air.

5. The apparatus of claim 4 wherein said primary air flow varying meansinclude a damper and said signal responsive means further include anactuator operatively connected to said damper and responsive to saidsignal from said light sensor for varying the position of said damper ina direction to maintain said sensed light at a desired valuecommensurate with a desired bed level.

6. The apparatus of claim 5 wherein said primary air is supplied to saidfurnace through a plurality of air ducts communicating with saidfurnace, each duct providing air to a particular associated region ofsaid furnace; each said duct includes a separate said damper therein;and a said sensing means is associated with each said air duct andassociated damper to control air supply to the said particularassociated region of said furnace.

7. The apparatus of claim 6 wherein each said air duct includes an airport at its downstream end and each said associated light sensing meansviews the interior of said furnace through said air port.

8. The apparatus of claim 7 wherein said furnace is a chemical recoveryfurnace; said material being burned includes a sodium compound and saidlight includes a wavelength characteristic of excited sodium atoms; andsaid sensing means is responsive only to said light wavelengthcharacteristic of excited sodium atoms.

9. The apparatus of claim 2 wherein primary air is supplied to saidfurnace for burning said material and comprises said control variable;and said signal responsive means include means for varying the flow ofsaid primary air.

10. The apparatus of claim 9 wherein said furnace is a chemical recoveryfurnace; said material being burned includes a sodium compound and saidlight includes a wavelength characteristic of excited sodium atoms; andsaid sensing means is responsive only to said light wavelengthcharacteristic of excited sodium atoms.

11. In a furnace in which a material is burned on a bed, emitting light,and in which the level of said bed is a function of the flow of primaryair supplied thereto, a method for controlling the level of said charbed comprising the steps of:

a. sensing only said light emanating from a particular zone in saidfurnace to provide a control signal which varies as a function of theintensity of said sensed light; and

b. varying the flow of said primary air as a function of said controlsignal.

12. The method of claim 11 wherein said zone is of limited verticalextent such that the intensity of said sensed light varies as a functionof the depth of said char bed.

13. The method of claim 10 wherein said air flow is varied inversely ofthe intensity of said sensed light.

14. The method of claim 12 wherein said material being burned includes asodium compound and said light includes a wavelength characteristic ofexcited sodium atoms and wherein said step of sensing said lightincludes sensing only that light having a wavelength characteristic ofexcited sodium atoms, whereby said control signal is indicative only ofthe intensity of said light wavelength characteristic of excited sodium

1. In a furnace in which a material is burned on a bed, emitting light,and in which the level of said bed is a function of a control variable,means for controlling the level of said bed comprising: a. means forsensing said emitted light and providing a signal which varies as afunction of the intensity of said sensed light and is indicative of thelevel of said bed; and b. means responsive to said signal forcontrolling said control variable to control said bed depth.
 2. Theapparatus of claim 1 wherein said sensing means sense light only from aparticular zone in said furnace such that the intensity of said sensedlight is related to the level of the bed with respect to the point ofmeasurement.
 3. The apparatus of claim 2 wherein said particular zone isof limited vertical extent within said furnace.
 4. The apparatus ofclaim 3 wherein primary air is supplied to said furnace for burning saidmaterial and comprises said control variable; and said signal responsivemeans include means for varying the flow of said primary air.
 5. Theapparatus of claim 4 wherein said primary air flow varying means includea damper and said signal responsive means further include an actuatoroperatively connected to said damper and responsive to said signal fromsaid light sensor for varying the position of said damper in a directionto maintain said sensed light at a desired value commensurate with adesired bed level.
 6. The apparatus of claim 5 wherein said primary airis supplied to said furnace through a plurality of air ductscommunicating with said furnace, each duct providing air to a particularassociated region of said furnace; each said duct includes a separatesaid damper therein; and a said sensing means is associated with eachsaid air duct and associated damper to control air supply to the saidparticular associated region of said furnace.
 7. The apparatus of claim6 wherein each said air duct includes an air port at its downstream endand each said associated light sensing means views the interior of saidfurnace through said air port.
 8. The apparatus of claim 7 wherein saidfurnace is a chemical recovery furnace; said material being burnedincludes a sodium compound and said light includes a wavelengthcharacteristic of excited sodium atoms; and said sensing means isresponsive only to said light wavelength characteristic of excitedsodium atoms.
 9. The apparatus of claim 2 wherein primary air issupplied to said furnace for burning said material and comprises saidcontrol variable; and said signal responsive means include means forvarying the flow of said primary air.
 10. The apparatus of claim 9wherein said furnace is a chemical recovery furnace; said material beingburned includes a sodium compound and said light includes a wavelengthcharacteristic of excited sodium atoms; and said sensing means isresponsive only to said light wavelength characteristic of excitedsodium atoms.
 11. In a furnace in which a material is burned on a bed,emitting light, and in which the level of said bed is a function of theflow of primary air supplied thereto, a method for controlling the levelof said char bed comprising the steps of: a. sensing only said lightemanating from a particular zone in said furnace to provide a controlsignal which varies as a function of the intensity of said sensed light;and b. varying the flow of said primary air as a function of saidcontrol signal.
 12. The method of claim 11 wherEin said zone is oflimited vertical extent such that the intensity of said sensed lightvaries as a function of the depth of said char bed.
 13. The method ofclaim 10 wherein said air flow is varied inversely of the intensity ofsaid sensed light.
 14. The method of claim 12 wherein said materialbeing burned includes a sodium compound and said light includes awavelength characteristic of excited sodium atoms and wherein said stepof sensing said light includes sensing only that light having awavelength characteristic of excited sodium atoms, whereby said controlsignal is indicative only of the intensity of said light wavelengthcharacteristic of excited sodium atoms.